It is known to provide a process for the dewatering of sludge in a centrifugal-force field whereby, in a first operating phase, the sludge is predried in a first force field in which the water is decanted from the solids and directly thereafter is subjected in a second working phase to a second force field serving to after dry the sludge. Apparatus for this purpose is also known and can use dewatering or sludge-concentrating or separating drums.
For example, German open application (Offenlegungsschrift) DT-OS No. 2 617 179 described an apparatus for this purpose in which the sludge and the liquid are passed through an outer drum in the same direction. In a second phase, i.e. in a second drum, the sludge is concentrated.
Another system for this purpose is described in German patent No. 976 357. In this system, an inner unperforated drum serves for preclarifying of the sludge and a disk serves as a weir over which the liquid flows and which is provided with bores through which the separated solids are removed. Outside of this disk, a worm is provided to advance the sludge and discharge the latter.
This discharge worm cooperates with an outer unperforated drum to carry the solids in the direction of the narrow end of the drum. Both drums are freely rotatable relative to one another and are driven with different angular velocities. In a preferred embodiment of this prior art system, the outer drum serves for after drying the solids displaced thereto by the discharge worm and is surrounded by a perforated drum.
Both of these conventional systems have a common disadvantage, namely, that they yield a sludge product having a high residual moisture content.
Decanting centrifuges operating in the afore-described manner for sludge dewatering have thus far proved to be unsatisfactory in many cases because decanting centrifuges yield a solid product whose residual moisture is unsatisfactory by comparison with dewatering systems using filter-band presses and stationary filter presses.
Investigations have already attempted to improve the effectiveness of decanting centrifuges by, for instance, a stop-and-go operation. In the stop-and-go process, the discharge worm is operated intermittently and the filling can proceed intermittently or continuously. In other words, during the discharge the apparatus can be further filled or feed of the sludge can be terminated.
The use of a perforated drum for after drying as described, for instance, in the aforementioned German patent No. 976 357, has been found to be especially ineffective for clarifier sludge since, even after a brief period of operation, a relatively dense zone of solids is formed and the liquid can no longer be forced through this zone. Thus, as soon as a dense zone of the aforedescribed type is formed, dewatering practically ceases and it is impossible to obtain a further reduction in the residual moisture content.